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Analyses of the Mechanical Design for a Towed Hydrophone
Array
Presented to the
University of California, San Diego
Department of Mechanical and Aerospace Engineering
MAE 199: Professor Jan Kleissl and Dr. Gerald D’Spain
06/13/2014
Prepared by:
Sulaman Ahmed
Spring 2014
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Abstract
To study the potential effects of man-made sound on odontocetes, particularly members
of the beaked whale family, a wide-band towed hydrophone array was designed and constructed.
Two candidate designs were created using the computer aided design program SolidWorks and
modeled in fluid flow using FloWorks. Results show that the Flooded Torpedo which had a drag
of 16 N, torque of 9 Nm, and a shear stress of 20 Pa, while the X-Array had a drag force of 52 N,
torque of 5 Nm, and a shear stress of 34 Pa. However, a modified X-Array with offset wing
struts was chosen to be built in order to provide aperture in 3 dimensions. In addition; the
Flooded Torpedo had no concrete method to allow water to flow through it, it had a higher
possibility of rotating, and the smaller distance between the hydrophones housed inside reduced
spatial resolution.
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Table of Contents
1. Introduction...........................................................................................................................4
2. Theory...................................................................................................................................5
3. Experimental Procedure.........................................................................................................6
4. Results...................................................................................................................................9
5. Discussion............................................................................................................................ 15
6. Conclusion........................................................................................................................... 16
7. Appendix A Surface and Cut plots......................................................................................... 18
8. Appendix B Tables................................................................................................................ 41
9. Appendix C Drawings........................................................................................................... 50
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1. Introduction
The objective of this project is to create a mechanical design for a towed passive
underwater acoustic array for detecting and localizing transient signals in the ultrasonic band,
greater than 20 kHz, from odontocetes, in particular members of the beaked whale family. The
array is designed to be deployed by two people, towed up to ten knots speed, resist rotating while
being towed, and hydrodynamically designed to minimize the noise on the hydrophones due to
turbulent pressure fluctuations. Two different designs were created, the “Flooded Torpedo” and
“X-Array”, and run through flow simulations to determine the final build design.
Ocean acoustics is the study of the underwater sound field in the ocean. Sound waves
travel underwater by pressure fluctuations that alternate between compressing and dilating the
water molecules. The sound waves radiate in all directions from the source. These pressure
changes caused by the sound waves can be detected by devices such as hydrophones.
Oceanographers are use hydrophones to study ocean sound, understand the sources, and
the potential effects of the sound on marine like, and the properties of the ocean environment.
Passive underwater acoustic monitoring has achieved better understanding of global warming,
earthquakes, movement of magma through the ocean floor from volcanic eruptions, and calls
from marine mammals.
San Diego is home to a large Navy base, which is equipped with vessels with active sonar
capabilities. Active sonar is an instrument used to image the underwater environment, by
emitting pulses of sound. These sounds are believed to be causing a disturbance in the behavior
of beaked whales. It is the goal of Southwest Fisheries Science Center and Scripps Institution of
Oceanography to determine the potential effects the Navy sonar and other man-made acoustic
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sounds has on the marine mammals in the proximity of the acoustic source operations.
2. Theory
It is not viable to use optics-based devices such video recorders except over short ranges
in the ocean because the distance light can travel is reduced significantly due to absorption. On
the other hand, sound waves can travel much greater distances than light. Sound travels about
five times faster underwater than in air. Hydrophones are microphones designed for use
underwater, and multiple hydrophones can be arranged in an array. Using an array, signals from
desired directions can be enhanced and signals from undesired directions can be ignored. A
signal processing technique such as beamforming uses the relative locations of the hydrophones
to estimate the location of the sound source. The ship typically must travel in a straight line to
collect a reliable data samples in order for the positions of the hydrophones to stay relatively
constant. The spatial resolution of the beamforming is affected by the relative distances between
the hydrophones; larger distances between hydrophones increases the resolution. For the project,
the array will be towed 300 meters behind the ship to minimize any disturbances to the
hydrophones due to both the noise radiated by the ship and array vibrations and turbulence
caused by the propulsion.
Drag in fluid mechanics is the force in the opposite direction of an object’s velocity,
acting to oppose the object’s motion through a fluid medium. Two main types of drag exist, skin
friction and pressure (form) drag. Skin friction drag is caused by the friction of the fluid and the
surface of the object flowing through it. Pressure drag is caused by the shape of the object and
how the boundary layer separates creating a pressure gradient. If a drag coefficient CD can be
estimated, drag can be calculated using the equation
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𝐹𝐷 =
1
2
𝜌𝜐2
𝐶 𝐷 𝐴⊥ (1)
In equation (1) ρ represents the density of the fluid, υ is the speed, and A is the cross-sectional
surface area. The coefficient depends on the shape of the object and the Reynolds number. The
Reynolds number is a non-dimension quantity estimated from the fluid properties, the speed, a
characteristic length. The Reynolds number is widely used to predict fluid flow patterns. The
power required to overcome drag is
𝑃𝐷 = 𝐹𝐷 𝑣 =
1
2
𝜌𝜐3
𝐴⊥ 𝐶 𝐷 (2)
A minimum amount of power must be supplied for a body to move. Shear stress is the force from
friction acting on the body of the object by the fluid it is moving through. The equation for shear
stress from drag force is
𝜏 =
𝐹 𝐷
𝐴∥
(3)
Where A now is the cross-sectional area parallel to the drag force. Shear stress is parallel to the
surface while normal stress is perpendicular. The stress on an object can create deformation or
make the object break in the areas of high stress. Stress is measured in units of Pascals, the same
units as for pressure.
3. Experimental Procedure
Two mechanical designs were modeled in Solidworks. The first one is called the Flooded
Torpedo Shell and the second design was named X-Array. Each model is designed to be towed
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300 meters behind a ship at a maximum speed of 10 knots. Once the designs were assembled in
SolidWorks, they were run through the simulation program FloWorks.
The Flooded Torpedo Shell is a two stage design, the interior stage housing the
hydrophones attached to the exterior shell of a piece of pipe. The electrical wires are spliced into
the towing cable which runs through the center. The interior stage is flooded with seawater to
allow the hydrophones to take measurements. This design allows for the hydrophones to stay in a
relatively secure area and minimizes any damage that can occur to them or other equipment.
Figure 1 illustrates a computer aided design (CAD) drawing of the flooded torpedo shell design.
Figure 1: Flooded Torpedo Shell
The X-Array is a four winged pipe with cross-section in the shape of the letter “X”.
These wings are streamline aluminum struts from Carlson Aircraft Inc. The wings begin from the
same location on the array body and extending out at an angle of 30 degrees from the pipe. The
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wings also are equally separated from each other, creating a shape that looks like a X-Wing
Starfighter from “Star Wars”. The hydrophones are mounted to the end of each of these wings
inside of capsules that are shaped to reduce drag. The towing cable can either run through the
pipe or be attached to the array through rings on the end. Since the array wing struts and
hydrophones capsules are freely flooded when deployed, the X-Array allows the hydrophones to
be unrestricted to the seawater by unlike the previous design, and improves accuracy of the
beamforming because the hydrophones are further spread apart, 0.5 meters between hydrophones
on opposite wings. Figure 2 is a CAD drawing of the X-Array design.
Figure 2: X-Array
A fluid flow simulation was done using the program FloWorks. Each design model was
run in the program using the same parameters. The parameters used in the simulation were the
external flow speed and direction with respect to the array, standard ocean temperature, and a
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velocity of 5 m/s (about 10 knots). The designs were run at angles offset from the array body
(pipe): 0 degrees (parallel to the array body), 1 degree, 2 degrees, 4 degrees, and 8 degrees.
Using the Pythagorean theorem, the angles to the body were created by changing the speed in the
vertical direction. So if the arrays were traveling at 5 m/s, zero degrees to the array body would
have a no velocity component in the vertical direction and 4 degrees would have a velocity of
0.351 m/s in the vertical direction. A surface parameters excel sheet, found in Appendix A
Surface and Cut plots, was created from the simulation’s solution along with several surface
plots, Appendix B Tables.
4. Results
The two models’ FloWorks simulation plots are presented in Appendices A Surface and
Cut plots and B Tables. In the flow simulations, the positive Z axis is the direction the array is
being towed. The Y axis is perpendicular to the surface. The appendices also include three more
models, based on the X-Array with new configurations of the wing placement, which were
created after the simulations of the Flooded Torpedo and original X-Array.
Parameter Minimum Maximum Average Bulk Average Surface area [m^2]
Pressure [Pa] -706895 529007 164165 1.25748
Temperature [K] 293.198 293.203 293.201 1.25748
Density [kg/m^3] 997.561 997.563 997.562 1.25748
Velocity [m/s] 0 0 0 1.25748
Shear Stress [Pa] 7.66431E-12 1673.18 19.9097 1.26585
Fluid Temperature [K] 293.198 293.203 293.201 1.25748
Normal Force [N] 2.67251 -0.53802 -0.1875 2.61106 1.26585
Shear Force [N] 13.758 -0.02342 0.08625 13.7577 1.26585
Force [N] 16.3787 -0.56144 -0.1013 16.3687 1.26585
Torque [N*m] 9.95017 6.1264 -7.839 0.151464 1.26585
Surface Area [m^2] 1.26585 -4.2E-05 -0.0001 3.3956E-06 1.26585
Torque of Normal Force [N*m] 2.52825 1.55811 -1.9882 0.106866 1.26585
Torque of Shear Force [N*m] 7.42317 4.5683 -5.8508 0.0445986 1.26585
Uniformity Index [ ] 1 1.25748
CAD Fluid Area [m^2] 1.28153 1.28153
Table 1: Flooded Torpedo Shell ( 0 degrees)
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Parameter Minimum Maximum Average
Bulk
Average
Surface area
[m^2]
Pressure [Pa] 96359.5 105142 100649 0.500327
Temperature [K] 293.2 293.203 293.203 0.500327
Density [kg/m^3] 997.562 997.562 997.562 0.500327
Velocity [m/s] 0 0 0 0.500327
Shear Stress [Pa]
2.92019E-
11 54.0495 33.7295 0.500327
Fluid Temperature [K] 293.2 293.203 293.203 0.500327
Normal Force [N] 35.9966 2.15824 -3.4519 -35.7657 0.500327
Shear Force [N] 16.8194 0.00468 0.00198 -16.8194 0.500327
Force [N] 52.7425 2.16293 -3.45 -52.5851 0.500327
Torque [N*m] 5.04599 -4.57561 1.65131 -1.34127 0.500327
Surface Area [m^2] 0.500327 5.4E-19 -7E-18 1.21973E-19 0.500327
Torque of Normal Force [N*m] 3.75924 -3.33369 1.10359 -1.34183 0.500327
Torque of Shear Force [N*m] 1.35734 -1.24192 0.54772 0.000560196 0.500327
Uniformity Index [ ] 1 0.500327
CAD Fluid Area [m^2] 0.593678 0.593678
Table 2: X-Array ( 0 degrees)
The surface plots for pressure show how the hydrodynamic pressure on the arrays varies
over the entire body. This information is important because of the resulting forces acting on the
array and because the hydrophones record the sound waves, which is a pressure wave. The plots
of array found in Appendix A Surface and Cut Plots and the tables found in Appendix B Tables
show the areas of high pressure and the fluctuations due to the array being towed which could
potentially disturb the hydrophones or in the worst case damage the array. In the Flooded
Torpedo, the area with the highest pressure is the front and end cones, which cover the cable and
seal off the midsection of the body. The X-Array also exhibits higher pressure on the area of the
cones and along the ridges of the wings. The pressure over the hydrophone casings, for the X-
Array is low in comparison. The average pressure experienced by the Flooded Torpedo and the
X-Array are 164 kPa and 101 kPa respectively. For the Flooded Torpedo, the pressure fluctuates
from -706 to 529 kPa and for the X-Array from 96 kPa to 105 kPa. The pressure fluctuations, the
minimum and maximum pressure, are much more significant in the Flooded Torpedo than in the
X-Array.
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The shear stress surface plots are useful to highlight the areas that will need attention
when the model will be assembled. These areas will have a higher possibility to break during use
and should be reinforced if necessary. The Flooded Torpedo will need care in the area where the
cones connect to the main body. The X-Array has high shear stress in the areas of where the
wing struts are attached. This concentration of shear stress can be problematic since all the wing
struts are attached in the same area of the body. The X-Array also has an average shear stress of
14 Pa higher than the Flooded Torpedo.
For the X-Array, the flow trajectories show the flow velocity around the hydrophones
does not reduce noticeably and for the Flooded Torpedo, the hydrophones are housed inside the
body so the water interacting with the hydrophones will be flowing as fast as the inlet and outlet
allow. Since the arrays are moving at constant velocity, the drag force will be equal to the net
force of the towing cable. From the surface parameters from Appendix B, the drag force on the
Flooded Torpedo is equal to about 16 N and for the X-Array the drag force is equal to about 52
N. The torque dictates how resistant to twisting the towing cables must be. If the cables cannot
resist the torque applied, the models will rotate while being towed. If they do rotate rapidly the
spatial information obtained from the beamforming may be miscalculated. The torque on the X-
Array is about 5 N and on the Flooded Torpedo is 9 N.
After further consideration, some tweaks were made to the original design of the X-Array
to create the first remodel, reference figure 3. The two wing strut pairs are now offset from each
other along the array body by a ½ meter. This change is helpful for the array design for several
reasons. Separation of the two pairs of struts along the main body of the array now provides
array aperture in the 3rd dimension, improving the directional information provided by the
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measurements. The stress at the base of each wing strut which was focused on one area of the
pipe has now been spread out, therefore removing the need to reinforce the pipe at this location.
To reduce the probability for the array to rotate, changing buoyancy of individual wing
struts will be considered to keep the array steady and upright when being towed. The second and
third remodels to the X-Array, figures 4 and 5, have increased the angle between the center pipe
of the array and the wing struts to 60 degrees. The two models differ only by a 90-degree
rotation about the array body.
Figure 3: X-Array Remodeled 1 (wings offset)
Parameter Minimum Maximum Average
Bulk
Average
Surface area
[m^2]
Pressure [Pa] 97209.5 103871 100959 0.480425
Temperature [K] 293.2 293.203 293.203 0.480425
Density [kg/m^3] 997.561 997.562 997.562 0.480425
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Velocity [m/s] 0 0 0 0.480425
Shear Stress [Pa] 1.38E-07 52.0174 30.2102 0.480425
Fluid Temperature [K] 293.2 293.203 293.203 0.480425
Normal Force [N] 42.5239 -13.6272 -17.765 36.1525 0.480425
Shear Force [N] 14.4792 -0.00416 0.00357 14.4792 0.480425
Force [N] 55.361 -13.6313 -17.761 50.6317 0.480425
Torque [N*m] 5.62004 -5.36516 0.28628 -1.64861 0.480425
Surface Area [m^2] 0.480425 -0.00083 -0.0009 4.31E-06 0.480425
Torque of Normal Force [N*m] 6.35543 -6.11573 0.51905 -1.64921 0.480425
Torque of Shear Force [N*m] 0.785834 0.75057 -0.2328 0.000603261 0.480425
Uniformity Index [ ] 1 0.480425
CAD Fluid Area [m^2] 0.606781 0.606781
Table 3: X-Array Remodeled 1 (wings offset)
Figure 4: X-Array Remodeled 2 (wider struts)
Parameter Minimum Maximum Average
Bulk
Average
Surface area
[m^2]
Pressure [Pa] 98189.1 103476 100791 0.512201
Temperature [K] 293.2 293.203 293.203 0.512201
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Density [kg/m^3] 997.561 997.562 997.562 0.512201
Velocity [m/s] 0 0 0 0.512201
Shear Stress [Pa]
1.30926E-
07 39.8513 15.9273 0.512201
Fluid Temperature [K] 293.2 293.203 293.203 0.512201
Normal Force [N] 98.2045 9.16301 -9.1747 -97.3446 0.512201
Shear Force [N] 8.14699 0.00032 0.00105 -8.14699 0.512201
Force [N] 106.286 9.16333 -9.1737 -105.492 0.512201
Torque [N*m] 9.68034 -8.04243 1.36951 5.21083 0.512201
Surface Area [m^2] 0.512201 -0.00063 -0.0004 0.000120831 0.512201
Torque of Normal Force [N*m] 9.31224 -7.62043 1.22535 5.21012 0.512201
Torque of Shear Force [N*m] 0.445944 -0.422 0.14416 0.000713627 0.512201
Uniformity Index [ ] 1 0.512201
CAD Fluid Area [m^2] 0.641655 0.641655
Table 4: X-Array Remodeled 2 (wider struts)
Figure 5: X-Array Remodeled 3 (orientation rotated 90 degrees)
Parameter Minimum Maximum Average
Bulk
Average
Surface area
[m^2]
Pressure [Pa] 98171.8 103143 100807 0.509449
Temperature [K] 293.2 293.203 293.203 0.509449
Density [kg/m^3] 997.562 997.562 997.562 0.509449
Velocity [m/s] 0 0 0 0.509449
Shear Stress [Pa]
2.12456E-
08 98.9349 22.4727 0.509449
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Fluid Temperature [K] 293.2 293.203 293.203 0.509449
Normal Force [N] 86.243 1.80078 -23.902 -82.845 0.509449
Shear Force [N] 11.4339 -0.00131 -0.2108 -11.4319 0.509449
Force [N] 97.3284 1.79946 -24.113 -94.277 0.509449
Torque [N*m] 10.3684 -10.2114 1.75987 -0.365351 0.509449
Surface Area [m^2] 0.509449 -0.00063 -0.0004 0.00012083 0.509449
Torque of Normal Force [N*m] 9.51814 -9.37847 1.58366 -0.362441 0.509449
Torque of Shear Force [N*m] 0.851402 -0.83296 0.17621
-
0.00291011 0.509449
Uniformity Index [ ] 1 0.509449
CAD Fluid Area [m^2] 0.641655 0.641655
Table 5: X-Array Remodeled 3 (wing position flipped)
The pressure fluctuations and the shear stress for the X-Array are reduced in the new
models in figure 4 and 5. The second and third remodeled of the X-Array do not differ much
between themselves in the results. They both experience only about half of the shear stress the
first remodel experiences. The pressure fluctuations are reduced by about 1 kPa. The torque is
almost the same as before and the drag force is increased to about 100 N.
5. Discussion
Although the simulations show that the Flooded Torpedo had a few better results in the
flow conditions, it was not enough to motivate further development. The largest hurdle the
Flooded Torpedo still had to encounter was the ability to allow water to enter and exit the model.
The two arrays were conceived to house hydrophones and record the most accurate data as
possible. A mechanism to allow water to flow in and out of the Torpedo Shell had not been
created yet. The disturbances from the entry and exit point could lead to significant degradation
in the hydrophones’ ability to record sound due to turbulence and flow noise. The accuracy for
the hydrophones for determining the sound source location depends on the distance between the
hydrophones in the array. The shell does not allow for significant distance in two directions
between the hydrophones. Also the Flooded Torpedo has higher torque then the X-Array by
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almost a factor of 2, thus allowing for significant rotation during towing. These elements gave
significant cause for the Flooded Torpedo Shell approach to be shelved.
Out of the several X-Array models, the second and third remodels (figures 4 and 5) gave
the best results. They were able to reduce the shear stress of the X-Array by half and the pressure
fluctuations were also reduced, allowing higher fidelity acoustic recording. The torque for the
two models was about the same as the original X-Array. The simulations also show that
changing the orientation between remodels 2 and 3 does not have a significant effect.
The casing for the hydrophones used in the X-Array models cannot be purchased at any
retailer and are costly to fabricate. A fused deposition manufacturing (FDM) technique is used
for these hydrophone casings. FDM is advantageous because of the reduced cost for most
complicated designs and the ability to produce intricate designs, such as a hollow sphere, that are
impossible for a machine shop to fabricate. FDM is an additive 3-D printing technology. The
printer lays plastic filament or metal wire down in layers building up the specified model. The
extrusion nozzle is fed the material from coils, as the plastic or metal enters the nozzle they are
heated up to liquid state and deposited. The plastic or metal instantly hardens after deposition,
forming the solid model. A computer aided manufacturing (CAM) software using
stereolithography file format (STL) controls the FDM printer. These files can be created directly
from SolidWorks drawings.
6. Conclusion
Two mechanical designs were modeled in SolidWorks and run through simulations in
FloWorks. The simulations showed that the drag on the X-Array was 36 N greater, 52 N to 16 N,
but the torque was half that of the Flooded Torpedo, 5 N to 10 N. The Flooded Torpedo has
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some better results in the simulations, but these results do not outweigh the problems: the
hydrophone placement being too close together, increased probability to rotate, and its inability
to allow water to flow in and out undisturbed. The X-Array design had refinements done to the
positions of the wing struts and this final design will be built with the aid of fused deposition
manufacturing (FDM) techniques. The engineering drawings for the design can be found in
Appendix C Drawings.
